Volume sensors are known which employ rotating toothed meter wheels which pass small quantities repeatedly, as defined by the transfer spaces between the teeth. A signal pulse for each small quantity is delivered by a magneto-electric sensor. The resulting measurement is simple, by addition of electric pulses, each assigned to a preset amount of liquid (German utility model 66 09 610, DE-AS 25 54 466, DE-PS 31 47 208).
Volume sensors of this kind are used in liquid-conveying conduits, and the gearwheels are driven by the flowing liquid. At high flow rates, particularly when using gearwheels having a small radius and with consequently small transfer quantities, there is a considerable increase in the flow resistance and a consequent throttling of the flow of liquid, which upwardly limits the measuring range. Another inevitable result hitherto has been considerable noise, which is extremely disturbing.
In known volume sensors (DE-PS 31 47 208) the differential magnetoresistors are inserted from the exterior into blind bores which are internally sealed from the measuring chamber by an insert of non-magnetic material. These non-magnetic inserts usually have a thickness of a few millimetres, so that a considerable air gap is left between the end faces of the teeth and the magnetoresistors. The insert also protects the differential magnetoresistor sensor, assumed to be very pressure-sensitive, against the pressure inside the volume sensor.
At high rotation speeds of the meter wheels, and also at elevated operating temperatures, which may be reached by the aforementioned volume sensors, it becomes more difficult to obtain a clear signal via a differential magnetoresistor sensor disposed in the region of the teeth.
An attempt has already been made to solve this problem by disposing one or more magnetically conductive pole pins in the non-magnetic insert (De-PS 31 47 208, EP 0 393 294 A1).